The present invention relates, in general, to the field of techniques for illuminating displays in electronic instruments. More particularly, the present invention relates to a backlighting technique for liquid crystal displays (LCDs) and other display devices in electronic instruments, for example, laser-based distance and speed measurement instruments.
The problem with backlighting for in-scope displays for rangefinding instruments intended for sports optics applications, for example, is that 85% light transmission of the scene may be desired leaving only 15% available for the display system. In actuality it may be even less than that. Therefore, the problem may be illustrated given a white wall in full sun has an effective illuminance of 100K to 120K nits where a “nit” is commonly defined as a unit of visible-light intensity used to specify the brightness of a liquid crystal display wherein one nit is equivalent to one candela per square meter.
With the above given display ratio of 85/15=5.67 one would require something closer to 500K nits (or on the order of 567-680K nits) to match the sunlight illuminance which cannot be done with currently available technology as there is no display bright enough to do that. However in practice, even a display illuminance of on the order of 100K nits exhibits good enough contrast under these conditions. As an example of currently available devices, a good organic light-emitting diode (OLED) display may be on the order of 40K nits while typical liquid crystal display (LCD) backlight displays are only around 6K nits with about 100K nits being what is required. In currently available laser rangefinders for sports optics applications a backlight LCD display may require about 64 milliamps of current to generate about 6K nits. Consequently, in bright sunlit conditions (like on a golf course) a special coating is required to block the visible illumination down by about a factor of six in order to make the display visible at 64 milliamps.